Gas turbines are used in many applications including gas-turbine-driven powerplants. The gas turbine exhaust has been used to improve the overall powerplant performance by taking the high temperature turbine exhaust and recovering the heat. It is normally recovered by conventional steam generating techniques, e.g., boiler, and routed to a steam turbine. The condensed steam may be recycled to the boiler. This arrangement is typically called a combined cycle powerplant. "Cogeneration" refers generally to the simultaneous on-site generation of electric energy and process steam or heat from the same plant. As described further below, as an aspect of the present invention, a cogeneration system is novelly incorporated into a Fischer-Tropsch-based system.
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch process was developed in early part of the 20.sup.th century in Germany. It was practiced commercially in Germany during World War II and has been practiced in South Africa for some time. An ongoing quest has existed, however, to improve the economics of the process.
The Fischer-Tropsch-based conversion process may first involving converting light hydrocarbons (such as methane) into synthesis gas (primarily CO and H.sub.2) and then converting the synthesis gas to heavier hydrocarbons through the Fischer-Tropsch reaction. The hydrocarbon products derived from the Fischer-Tropsch reaction range from some methane to high molecular weight paraffinic waxes containing more than 50 carbon atoms, but primarily includes C5+.
Numerous Fischer-Tropsch catalysts, such as iron and cobalt catalysts, have been used in carrying out the reaction, and both saturated and unsaturated hydrocarbons can be produced. Numerous types of systems and reactors have been used for carrying out the Fischer-Tropsch reaction. See, for example, U.S. Pat. Nos. 4,883,170 and 4,973,453, which are incorporated herein by reference for all purposes.
It has been a quest for some time to improve the economics of hydrocarbon conversion systems, particularly those utilizing the Fischer-Tropsch reaction. Improved economics will allow wide-scale adoption of the process in numerous sites and for numerous applications. These efforts are reflected in U.S. Pat. Nos. 5,733,941 and 5,861,441, which are incorporated herein by reference for all purposes.
One particular challenge to improved performance is related to the thermal limitations of gas turbines that are included in some embodiments of Fischer-Tropsch systems. The turbine blades can only withstand a certain level of heat. This in turn limits the throughput since the oxygen-containing gas supplied to the turbine expander must be limited to control the temperature.